1. Field of the Invention
This invention relates to methods of crosslinking polysaccharides to form crosslinked polysaccharides, and more particularly, to crosslinked polysaccharides that may be incorporated in fluids useful in, for example, applications requiring a crosslinked viscoelastic gel.
2. Description of the Prior Art
Polysaccharides are carbohydrates in which tens, hundreds, or even thousands of simple sugars are linked together. Since they have no free anomeric hydroxyls (except for one at the end of the chain), they are not reducing sugars and do not show mutarotation. Cellulose, starch, and various galactomannans are some of the most-widely occurring polysaccharides. Polysaccharides are commonly used to viscosify aqueous fluids to create, e.g., viscosified treatment fluids that inhibit particle settling by virtue of viscosity. If the polysaccharide is crosslinked, these viscosified treatment fluids can approach near zero particle settling rates.
Such viscosified treatment fluids are often used in industries such as the mining, explosive, and petroleum industries. For instance, in petroleum recovery applications such as drilling, hydraulic fracturing, and gravel packing, such fluids are used to carry particulates into a subterranean formation for various purposes. In hydraulic fracturing operations, viscosified treatment fluids are used to stimulate the formation. The viscosified fluid carries and deposits a propping agent through the wellbore and into fractures in the formation. Once the desired deposition has occurred, the viscosity of the treatment fluid is reduced, the treatment fluid is returned to the surface, and the proppant is left in the formation, inter alia, to maintain the integrity of the fracture.
In gravel packing operations, a pack of gravel is placed on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation. The resultant structure, inter alia, presents a barrier to migrating formation fines but allows formation fluids to flow to the well bore. The gravel is carried to the formation in the form of a slurry by mixing gravel with a viscosified treatment fluid. Once the gravel pack is placed in the wellbore, the viscosity of the treatment fluid is reduced, and it is returned to the surface.
To increase the viscosity of the viscosified fluid, the polysaccharide component of the fluid may be crosslinked. Conventional crosslinking agents comprise aluminum, titanium, zirconium, and boron. The crosslinking behavior of these conventional crosslinking agents, however, may be difficult to control, and the resultant viscosity of the fluid may be unpredictable. Being able to predict how the crosslinking will occur provides for control of the ultimate viscosity of the treatment fluid. The primary means of trying to control the crosslinking behavior of these conventional agents involves varying the concentration of the crosslinker in the composition and the pH of the composition at a given temperature, which can be subject to great variability and human error. For example, if too much crosslinking agent is added or if the pH is too low or too high, then the viscosity of the treatment fluid may be adversely affected. Even employing such techniques has not provided the desired level of predictability for the ultimate viscosity of the treatment fluid.
Also, the crosslinking behavior of such conventional crosslinking agents may become inhibited by components in the treatment fluid. For example, the composition of the water component of an aqueous treatment fluid can interfere with the crosslinking behavior of conventional crosslinking agents. Moreover, such conventionally crosslinked polysaccharides may be heavily dependent on conditions such as pH and temperature. Because of this dependency and resultant instability, the viscosified treatment fluid may lose its viscosity prematurely, for instance, the proppant or gravel can drop out of a conventional viscosified treatment fluid before it has been placed in the desired interval in the subterranean formation neighboring the well bore.